1. Field of the Invention
The present invention relates generally to the use of alginates in water purification systems. More particularly, the present invention involves using doped alginates to remove oxyanions such as arsenic (V) and selenium (IV) oxyanions from polluted water.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and grouped in the appended bibliography.
Of the many toxic materials that may be present as contaminants in natural waters and wastewaters, a few (specifically arsenic (V), chromium (VI), and selenium (IV) occur as oxyanions. Removal of arsenic is, in particular, an issue of increasing concern. Reevaluation of the current Maximum Contaminant Level (MCL) for arsenic is mandated under the 1996 re-authorization of the Safe Drinking Water Act. A decrease in the current standard (50 .mu.g/l) would impact the required levels of treatment not only for potable water, but also for hazardous wastewaters and effluents under the Resource Conservation and Recovery Act.
Arsenic is a major contaminant of concern at many Superfund sites (1, 2). Arsenic containing wastestreams are generated in the microelectronics industry due to the use of arsenic in the form of gallium arsenide (GaAs) for the manufacturing of semiconductor devices (4). The average concentration of arsenic in wastewater at facilities which produce GaAs has been estimated to be 2.4 mg/l (5).
For arsenic-contaminated wastestreams, various physical/chemical treatment technologies have been applied for contaminant removal. Arsenic-contaminated water collected during clean up at a former pesticide facility was treated in a full-scale process involving co-precipitation/adsorption with ferric chloride, filtration and carbon adsorption (6).
In pilot-scale studies using electrochemically generated Fe.sup.2+ and hydrogen peroxide, arsenic was efficiently removed from contaminated waters at both a Superfund site and a wood-preserving facility (1). Other processes, such as adsorption onto activated carbon (7, 8) or fly-ash (9) have been tested at bench scale.
Many of these processes generated a significant quantity of sludges or other solid wastes. In some cases, these materials had to be disposed of as hazardous waste. In some applications, selective removal of oxyanions may provide options for more economical treatment of wastestreams. For example, by reducing subsequent sludge disposal costs.
At the bench scale, biosorbents have been tested primarily for removal of heavy metal cations, which are removed preferentially to alkali and alkaline earth metals and anionic species (10). Alginic acid has been shown to be effective at removing many cationic metals from solution including Pb.sup.2+ and Cu.sup.2+ (11), UO.sup.2+.sub.2 (12). Cu.sup.2+ (13), Cu.sup.2+, Zn.sup.2+, Cd.sup.2+, and Ni.sup.2+, (14), Nd.sup.3+ and Yb.sup.3+ (15), and .sup.226 Ra (16).
Pretreatment or doping of an anionic biosorbent, such as alginic acid, with cations allows (indirect) interaction between the biosorbent and anionic contaminants. Charged polysaccharides, such as sodium alginate, often form hydrogels in the presence of cations (18), the gel characteristic depend on the specific cation. Metal recovery (Co, Cu, As, Fe, Mg, Al, Ca) from an acidic (pH 2-3) cobalt ore leachate with alginic acid has been investigated by Jang et al. (17).
As is apparent from the above, there is a continuing need to provide efficient and economical systems for purifying polluted waters. Such systems are especially needed for removing hazardous oxyanionic pollutants, such as arsenic and selenium oxyanions.